It's your obligation to check and revert if your application/website support PHP 5.3 (which in most cases does support 5.3)

You may refer to http://www.php.net/manual/en/migration53.changes.php for changes in PHP 5.3 New feature can be found at : http://www.php.net/manual/en/migration53.new-features.phpIncompatible backward can be found at: http://www.php.net/manual/en/migration53.incompatible.php